1. Field of the Invention
This invention relates generally to a method for the determination of the degree of magnification or minification or minimization of radiographic images. The invention relates more specifically to a method and apparatus capable of facilitating the accurate calculation of the degree of magnification or minification or minimization of radiographic images.
2. Discussion of the Prior Art
In the interpretation of radiographic images, various approaches are known for differentiating between structures which appear in such images. For example, U.S. Pat. No. 2,109,780 describes a shoe fitting device in which a marker of an opaque material which is impermeable to x-ray radiation is used to distinguish between the fleshy parts of the foot and the contiguous parts of a shoe upper.
Furthermore, various approaches are known which serve to locate anatomical structures in the radiographic images of the human body. For example, U.S. Pat. No. 4,506,676 describes an adhesive, flexible tape which includes radiopaque dots. The tape is applied to a patient to identify an area of interest, and the image of the dot appears in the x-ray photograph as a pointer to the area.
U.S. Pat. No. 3,812,842 describes an indexing scale which includes markers that are visible in x-ray photographs. The markers facilitate locating a particular blood vessel relative to the scale in the x-ray photograph, and the arms which support the indexing device are secured to a patient""s chest with adhesive tape.
U.S. Pat. No. 5,394,457 describes a device for marking body sites for imaging medical examinations. By using multiple markers, two x-ray exposures taken from different directions can be used to determine the spatial location of any desired point. The marker has a head with a cavity for receiving a substance which exhibits high contrast, and in one embodiment, the fixture which supports the marker is attached to the skin with an adhesive.
U.S. Pat. No. 5,469,847 describes an adhesive surface marker with a cavity for receiving imaging materials. The surface marker contains an x-ray opaque gel, such as barium sulfate, which is sealed on all sides by an outer casing. The use of multiple markers provides reference points which allow the geometrical calculation of the precise location of a particular site within a patient""s body.
The assessment of radiographic images, however, must account for image distortion caused by parallax and magnification or minification or minimization (hereinafter referred to as xe2x80x9cmagnificationxe2x80x9d) artifact. That is, magnification artifact introduces a source of error into the measurement of linear distances in radiographic images. Thus, while use of an apparatus such as that described in U.S. Pat. No.5,469,847 will allow the geometrical calculation of the precise location of a particular site, the image of the distance from any one marker to another marker is always magnified to some extent.
U.S. Pat. No. 3,941,127 describes a device that is placed in the same plane as the anatomical object of interest in order to correct for magnification distortion. A radiopaque interval marker reference is placed adjacent to the torso of a patient at a distance from the film so as to make equal magnification distortion between the markings on the reference and the plane of the patient""s torso.
To determine the actual distance from one location in an image to another location in an image, however, the true degree of magnification must be known. One approach to providing a method of calculating the degree of magnification employs an apparatus with two small lead spheres encased in plastic and spaced 100 mm apart (made by Osteonics, Inc., Allendale, N.J.). Although Osteonics discloses the possible necessity of wrapping one or both ends of the apparatus with led foil to improve the metallic sphere visibility on an x-ray, the device has neither an attenuating plate, nor adhesive to facilitate attachment of the device to a patient. In such a two-point, but one-dimensional device, the potential exists for underestimation of the magnification artifact. If the two points are not in a plane exactly parallel to the radiographic film, the apparent distance between the spheres will decrease and appear shorter on the image, thereby making it impossible to accurately calculate the degree of magnification.
In order to overcome the limitation associated with the aforementioned two-point marker, an apparatus employing a flat plate of standard metal with three small lead spheres the size of a BB pressed into 1.5 mm holes spaced in a 40 mm equilateral triangle configuration (made by Ortho-Graphics, Inc., Salt Lake City, Utah) is described in Stevens, P. M., xe2x80x9cRadiographic Distortion of Bones: A Marker Study,xe2x80x9d Orthopedics, Vol. 12, No. 11 (1989), pp. 1457-1462. Though acknowledging the hypothetical superiority of a spherical marker, the reference dismisses it as being both too heavy and awkward to secure to a patient, and capable of casting a shadow with a fuzzy perimeter, making measurement imprecise. While one side of the three-point device has a 90xc2x0-bend with a 1 cm flat surface that is applied to a patient, the device has no self-adhesive, and is very difficult to attach. Though the flat plate does help to sharpen the image of the small spheres, the three points are all located in the same plane. As with the aforementioned two-point device, if the three points are not all in a plane exactly parallel to the radiographic film, the apparent distance between the spheres will decrease and appear shorter on the image, thereby making it impossible to accurately calculate the degree of magnification.
Thus, while each of the latter devices may provide an assessment of the distortion in radiographic images, because the two or three points are all located in one plane, neither of these devices is capable of providing a truly reliable, accurate assessment of the degree of magnification. Furthermore, while the three-point device may be directionally somewhat more accurate than the two-point device, the inability to easily and quickly attach it to and detach it from a patient diminishes its attractiveness.
Therefore, a general need exists to provide the radiographical profession with a method for the accurate determination of the degree of magnification of radiographic images. A more specific need exists for a method and apparatus capable of facilitating the accurate calculation of the degree of magnification of radiographic images which is both less sensitive to the orientation of the marker relative to the beam of radiation, and easily mounted on and detached from a subject.
It is an object of the present invention to provide a method and apparatus capable of facilitating the accurate calculation of the degree of magnification of radiographic images (including digital and analog radiographic images). The description below is generally in the context of developed analog radiographic images made on X-ray film with a conventional X-ray machine but it will be understood that the images would be essentially the same if the images were made digitally using a digital radiation detector also known as a digital X-ray machine.
Accordingly, the present invention advantageously relates to a radiographic reference marker which satisfies each of the aforementioned criteria. In a preferred embodiment, the reference marker comprises a reusable portion and a disposable portion. The reusable portion comprises a radiopaque sphere of outside diameter d1, and a radiolucent plastic housing capable of engaging the sphere and comprising a plate attaching face and a housing attaching face. The plate attaching face comprises in a center portion thereof an aperture of a diameter capable of allowing passage therethrough of the sphere, and the housing attaching face comprises a first portion of a means for removably attaching the reusable portion to the disposable portion. The reusable portion also comprises an attenuating plate having a first surface and a second surface, a thickness sufficient to attenuate radiation, and a means for removably attaching the second surface of the attenuating plate to the plate attaching face of the plastic housing. The disposable portion comprises a mounting substrate having a substrate attaching face and a mounting face, and the substrate attaching face comprises a second portion of the means for removably attaching the reusable portion to the disposable portion. The mounting face comprises a means for removably attaching the mounting substrate to a subject.
The invention further relates to a radiographic reference marker which facilitates the accurate calculation of the degree of magnification or minification or minimization of radiographic images which includes a reusable portion and a disposable portion. The reusable portion includes a radiopaque sphere of outside diameter d1, and a radiolucent plastic housing capabable of frictionally but removably engaging the sphere and including a plate attaching face and a housing attaching face. The plate attaching face includes in a center portion thereof an aperture of a diameter capable of allowing passage therethrough of the sphere, and the housing attaching face includes a first portion of a means for removably attaching the reusable portion to the disposable portion. The reusable portion also includes an attenuating plate having a first surface and a second surface, a thickness sufficient to attenuate radiation, and a means for removably attaching the second surface of the attenuating plate to the plate attaching face of the plastic housing. The disposable portion includes a mounting substrate having a substrate attaching face and a mounting face, and the substrate attaching face includes a second portion of the means for removably attaching the reusable portion to the disposable portion. The mounting face includes a means for removably attaching the mounting substrate to a subject. The reference marker is employed by attaching the reusable portion to the disposable portion, affixing the attached reusable portion and disposable portion to a subject, and irradiating a radiographic film to produce a first image of diameter d2 and a second shadow image disposed concentrically around the first image. Once diameter d2 has been measured, comparison of measured image diameter d2 with known sphere diameter d1 enables one to accurately calculate the degree of the magnification or minification of the radiographic image.
The invention further relates to a method of calculating the degree of magnification of radiographic images. The reference marker is employed by affixing the disposable portion to a subject, attaching the reusable portion to the disposable portion, and irradiating a radiographic film to produce both a first image of outside diameter d2 and a second shadow image, i.e., a graying effect, disposed concentrically around the first image. Once diameter d2 has been measured, comparison of measured image diameter d2 with known sphere diameter d1 enables one to accurately calculate the degree of magnification of the radiographic image.
The advantages associated with the present reference marker are numerous. First, by virtue of its use of a spherical reference point, it overcomes the limitation associated with one-dimensional devices, and provides for the accurate calculation of the degree of magnification of an image. By employing the attenuating plate in conjunction with the uniform plastic housing, the marker produces the uniform secondary shadowing around the more dense material of the sphere. Finally, by virtue of its self-adhesive means for attaching to a subject, the marker is easily mounted on and detached from a subject.